1. Field of the Invention
The field of the invention is that of display devices and in particular that of flat television screens of large dimension, consisting of an active liquid crystal matrix with matrix addressing (LCD).
2. Discussion of the Background
Currently, the construction of flat television screens of large dimensions (thickness less than 5 cm and surface area greater than 1 m.sup.2) constitutes a priority objective in the presentation of high-definition TV colour images. The applications of these screens are extremely varied and cover both the professional sector and the general-public sector.
Hitherto, television images of large dimensions have been produced by projecting the image onto a screen, from a small-size liquid crystal matrix (diagonal less than 5 cm). Several companies are now constructing and marketing front-projection or retroprojection systems. However, these systems are of limited performance in view of the following parameters:
the bulkiness of the overall structure: projection plus screen; PA1 the low optical efficiency of a projection system; PA1 insufficient luminance of the projected image when the latter is of large dimension. PA1 it comprises the employing of an array R.sub.1 of lenses on the rear face of the substrate intended for the construction of the spatial modulator; PA1 the various masks are imaged directly through the array R.sub.1 of lenses so as to produce, on the front face of the substrate, the elementary pixels, the electrodes and the control circuits.
Under these conditions, it would be particularly beneficial to work towards a truly flat screen with active liquid crystal matrix, of large size and viewed directly with no projection device.
Nevertheless, the extension of current technologies in large-size active liquid crystal matrices is essentially limited by the dimension of the elementary pixels of the said active matrices, necessary in the construction of the flat screen.
Thus, when pixel sizes of the order of a mm.sup.2 are envisaged, such an area of the liquid crystal electrooptical element leads to a considerable capacitance of the pixel. This considerable capacitance hampers the operation of the large-size screen at the video rate. Currently, all of the points of a line (around 800 points) can be addressed in 60 .mu.s, this being made possible by virtue of the small size of the pixels (100 .mu.m.times.100 .mu.m). Such addressing becomes difficult when the size of the elementary pixels is increased beyond this dimension.